In recent years, with a remarkable spread of a mobile phones, a cordless phones, a patch antennas for a RFID, a lens antennas such as a radio telescope, a millimeter-wave radar, and the like and significant progress of a satellite communication apparatus, a communication signal is demanded to have a higher frequency, and communication apparatus is demanded to be more compact. As the dielectric constant of a material for an antenna incorporated inside the communication apparatus becomes higher, the communication signal is allowed to have a higher frequency, and the communication apparatus is allowed to be more compact. The dielectric constant and the dielectric loss tangent are parameters indicating the degree of a polarization inside a dielectric and the loss of an energy generated by a polarization inside the dielectric or by increasing conductivity thereto respectively. Therefore the use of the material for the antenna having a high dielectric constant allows the communication signal to have a higher frequency, circuits to be shorter, and the communication apparatus to be more compact. In addition, as the use form of the communication apparatus diversifies, the material for the antenna is demanded to have a low degree of a change in its electrical characteristic at low to high temperatures and be excellent in its flame-retardant property.
As a material for obtaining an antenna showing a high dielectric constant and having a low dielectric loss tangent in a wide temperature range from a low temperature to a high temperature, a dielectric elastomer composition is known. The dielectric elastomer composition contains the elastomer such as ethylene propylene rubber mixed with the barium•neodymium ceramic powder having a temperature coefficient α (unit: 1/° C.) of the dielectric constant is in the range of (−200 to 100)×10−6 in the temperature range from −40° C. to 100° C. (see patent document 1).
As a measure for improving the flame-retardant property of the material for the antenna, it is known that the material for the antenna contains a halogenated flame retardant such as a brominated flame retardant or a chlorinated flame retardant, for example, polybromodiphenyl ether (hereinafter referred to as PBDE), polybromobiphenyl (hereinafter referred to as PBB). To improve the flame-retardant property of the brominated flame retardant other than the PBB and the PBDE, normally the brominated flame retardant is used in combination with antimony trioxide.
It is known that to improve the flame-retardant property of the elastomer material, in addition to the halogenated flame retardant, the elastomer material contains a metal hydroxide, expanded graphite. It is known that the metal hydroxide is contained in the elastomer material composing a transfer belt and the like of an electrophotographic device (see patent document 2).
The dielectric elastomer composition disclosed in the patent document 1 has a low degree of a change in its electrical characteristic at low to high temperatures and an excellent dielectric characteristic but does not contain the flame retardant. Therefore the dielectric elastomer composition disclosed in the patent document 1 cannot be used for applications demanded to have the flame-retardant property.
In using the halogenated flame retardant as the flame retardant to improve the flame-retardant property of the dielectric elastomer composition, there is a fear that at the time of disposal, dioxin is generated from the halogenated flame retardant. Thus the use of the halogenated flame retardant is environmentally unpreferable. The PBB and the PBDE cannot be used for electrical and electronic products in accordance with the RoHS directive (Restriction of the use of certain hazardous substances in electrical and electronic equipment) established by the European Union (EU) in January of 2003.
Although the use of the brominated flame retardant other than the PBB and the PBDE is not prohibited, as described above, the brominated flame retardant other than the PBB and the PBDE is used in combination with the antimony trioxide. The use of the antimony trioxide is unpreferable because the antimony trioxide contains a slight amount of lead, mercury, hexavalent chrome, and cadmium as impurities.
When the metal hydroxide is used as the flame retardant, it is difficult to obtain the flame-retardant effect unless the elastomer material contains a large amount of the metal hydroxide. When the elastomer material contains a large amount of the metal hydroxide, generally the dielectric loss tangent becomes high. Thus it is not known to use the metal hydroxide for the material of the antenna or the like in which a low dielectric loss tangent is demanded.
In the case where the material for the antenna contains the expanded graphite, the flame-retardant property thereof is improved, but the dielectric characteristic thereof extremely deteriorates. Thus the use of the expanded graphite is unpreferable.
Patent document 1: Japanese Patent Application Laid-Open No. 2006-1989
Patent document 2: Japanese Patent Application Laid-Open No. 2005-97493